A satellite is often used to relay signals to their destination. This allows the transmitted signal to reach beyond the distance constraint imposed by the curvature of the earth. The transmitted signal is modulated to a frequency compatible with the satellite receiver. The satellite receives this signal and then typically re-modulates the signal to a different frequency before it is retransmitted back to the earth. This re-modulation prevents interference between the earth-based receiver and earth-based transmitter.
The signal often contains digital information. This digital information is often directly modulated onto the carrier. The frequency with which the bits are modulated onto the carrier is known as the bit frequency. Alternatively, when Direct-Sequence Spread Spectrum Code Division Multiple Access (DSSS CDMA) is used, the information is spread prior to transmission, and this spread information is modulated at the chip frequency which is typically significantly higher than the information bit rate.
When a satellite is used which is not in a geosynchronous orbit, the signal received at the earth-based receiver is shifted from the frequency that was transmitted due to the motion of the satellite with respect to the earth. In addition, the bit rate and chip rate are also shifted. This change in frequency is referred to as the Doppler frequency shift. This shift is significant when low earth orbits, medium earth orbits and highly elliptical orbits such as the Molniya orbit are used.
The Doppler shift of both the carrier frequency and the bit frequency must be compensated for at the earth-based receiver in order to demodulate the signal. In the current state of the art, several methods are used to compensate for the Doppler shifts.
Classical carrier and bit/chip rate search algorithms which sweep the carrier and bit/chip rate in order to acquire the signal are often used. These receivers search over the entire range of possible received frequencies. Receivers employing this approach suffer from long acquisition times. This is especially evident in CDMA systems where a small frequency error will cause the demodulator either to not acquire or to falsely acquire.
When long acquisition times cannot be tolerated, as in the case for Time Domain Multiple Access (TDMA) and burst signals, long preambles containing unmodulated carrier frequency and known bit patterns are often employed. These approaches thus suffer from poor bandwidth efficiencies as these preambles become increasing longer as the frequency uncertainty increases.
Finally, rapid acquisition receivers often employ parallel channels, each attempting to acquire the signal over a portion of the frequency uncertainty range. This approach increases complexity and cost due to the additional hardware for each channel.
If any of the previously described approaches does not accurately compensate for the frequency offsets, the performance of the system is degraded.
Thus what is needed is a method for compensating for Doppler frequency shifts in satellite communication systems, where the method can rapidly acquire the signal while maintaining bandwidth efficiency, without requiring complex and costly equipment and while minimizing degradation in performance.